The invention relates generally to implantable medical devices, such as cardiac pacemakers, cardioverters and defibrillators. More particularly, this invention relates to lead systems employed for pacing, cardioverting or defibrillating a heart.
Implantable stimulation devices (ISDs), such as cardiac pacemakers, are often used to remedy improper heart function. These devices generally provide an electrical pulse to a selected area of the heart that is not (in terms of timing or strength) adequately receiving its natural pulse. Under abnormal cardiac conditions, and particularly cardiac rhythm disturbances, pacemaker therapy is applied to remedy several forms of cardiac arrhythmias (rhythm disturbances) including bradycardias, AV conduction block, supraventricular tachycardias, and atrial and ventricular ectopic arrhythmias.
There are essentially two kinds of pacemakers: single-chamber and dual-chamber. A single-chamber pacemaker is capable of sensing and pacing in only one of the atrium or the ventricle. From a practical standpoint, there are essentially two forms of single-chamber pacing: VVI (senses and paces in the ventricle) and AAI (senses and paces in the atrium).
A dual-chamber pacemaker is capable of sensing and pacing in both the atrium and the ventricle. There are many modes of dual-chamber pacing such as VDD (paces in the ventricle only, senses in the atrium and ventricle), DVI (paces in the atrium and ventricle, and senses in the ventricle only), DDI (senses and paces in both the atrium and ventricle), and DDD (senses and paces in both the atrium and ventricle, with an inhibited and triggered response to sensing).
A letter xe2x80x9cRxe2x80x9d is sometimes added to these pacemaker modes to indicate the pacemaker""s ability to provide rate-modulated (also sometimes called rate-responsive or rate-adaptive) pacing in response to input from an independent sensor. For instance, a DDDR pacemaker is capable of adapting to the need to increase a patient""s heart rate in response to physiologic stress in the absence of intrinsic response from a patient""s sinus node.
A pacemaker uses a lead system to perform its sensing and stimulation functions. A lead system typically comprises at least one lead, one or more conductor coils, and one or more electrodes. The lead is the insulated wire used to connect the pulse generator of a pacemaker to the cardiac tissue. The lead carries the output stimulus from the pulse generator to the heart and, in demand modes, relays intrinsic cardiac signals back to the sensing circuitry of the pacemaker. Typically, a single-chamber pacemaker requires one lead, whereas a dual-chamber pacemaker requires two leads (one for the atrium and another for the ventricle). The conductor coil is the internal core of the pacing lead through which current flows between the pulse generator and the electrodes.
A lead may be unipolar or bipolar. A unipolar lead is a pacing lead having one electrical pole external to the pulse generator, which is usually located in the heart. The unipolar lead has one conductor coil. The electrical pole is typically a stimulating cathode (i.e., negative pole) at the distal tip of the lead. As used herein, a distal end of the lead is the end which is farther away from the pacemaker. A proximal end of the lead is the end which is connects to the pacemaker. The cathode is the electrode through which a stimulating pulse is delivered. The anode (i.e., positive pole) is typically attached to the case, or housing, of the pacemaker. A stimulating pulse returns to the anode using the body tissue as a return current path. A unipolar lead is relatively small in size and is theoretically more reliable than a bipolar lead. However, a unipolar lead/pacing system is more susceptible to interference by other electrical activity in a patient""s body, such as inhibition due to myopotentials, and further may be prone to pectoral stimulation.
On the other hand, a bipolar lead is a pacing lead with two electrical poles that are external to the pulse generator. The bipolar lead has two conductor coils. The stimulating cathode is typically at the distal tip of the pacing lead, while the anode is an annular (i.e., ring) electrode which is few millimeters proximal to the cathode. As such, bipolar leads are less prone to pectoral stimulation. A bipolar lead has better signal-to-noise ratio than that of a unipolar lead, and thus, is less susceptible to interference from myopotential inhibition.
In practice, the cathode (i.e., stimulating) electrode is typically placed in contact with the heart tissue in order to stimulate the cardiac tissue. The anode electrode, however, does not need to be in contact with the heart tissue, since blood tends to conduct electrical currents better than the tissue itself. Nonetheless, it is preferable to have the sensing electrode in contact with the heart tissue to allow the detection of more distinct signals. As used herein, the terms xe2x80x9ccathodexe2x80x9d and xe2x80x9canodexe2x80x9d are relative by definition. An anode electrode is one which is more positive than a cathode electrode. A cathode electrode is one which is more negative than an anode electrode. For more details on bipolar lead structure and electrode placement, reference is made to commonly-assigned U.S. Pat. No. 5,522,855 (Hoegnelid), issued Jun. 4, 1996, and is incorporated herein in its entirety by reference.
Recently, combination sensing, pacing and defibrillation bipolar (xe2x80x9ccombipolarexe2x80x9d) leads were introduced to reduce the number of conductors implanted in the heart. More particularly, a unipolar atrial lead may be used with a ventricular tip electrode (placed in the apex region of the right ventricle) to perform bipolar sensing in the atrium. Atrial signals are often weak. Accordingly, interference from muscular activity typically causes problems in measurements taken in the atrium. Bipolar electrodes have often been used in the atrium in order to minimize the effects of such muscular interference. In this case, with the ventricular tip electrode as the second electrode, there is a substantial possibility of sensing myopotentials greater than that of a standard bipolar atrial electrode. Moreover, atrial pacing is still performed in a unipolar fashion and is, thereby, still susceptible to interference by other electrical activity in a patient""s body. For more information on combipolar sensing, reference is made to commonly-assigned U.S. Pat. No. 5,571,143, issued to Hoegnelid et al., and is incorporated herein in its entirety.
Therefore, there is a need in the cardiac pacing technology to sense and pace the atrium using a unipolar lead while maintaining bipolar modality. Such lead structure should minimize the complexity and number of conductors implanted in the heart.
To overcome the limitations of the prior art, the invention provides a system and method for performing bipolar sensing and pacing in a heart. In one embodiment, the invention provides a lead system which performs bipolar pacing in an atrium of the heart. The lead system comprises an atrial electrode configured for placement in the atrium, and a ventricular electrode configured for placement in the ventricle and having a surface area greater than about 60 mm2. In another embodiment, the lead system comprises an atrial electrode configured for placement in the atrium, and electrically connectable to a pacing circuit. The lead system further comprises a defibrillation electrode configured for placement in the heart, and electrically connectable to the pacing circuit. In another embodiment, the invention provides a lead system which performs bipolar sensing in an atrium of the heart. The lead system comprises an atrial electrode configured for placement in the atrium, and a defibrillation electrode configured for placement in the heart.
Furthermore, the invention provides a method of performing bipolar pacing in an atrium of the heart. The method comprises the steps of delivering pacing signals to the heart via an atrial electrode placed in the atrium, and returning the delivered pacing signals to a pacing circuit via a return electrode having a surface area greater than about 60 mm2. In another embodiment, the method comprises the steps of delivering a pacing signal via an atrial electrode placed in the atrium, and returning the delivered pacing signal via a defibrillation electrode. In another embodiment, the invention provides a method of performing bipolar sensing in an atrium of the heart. The method comprises the steps of sensing an atrial signal via an atrial electrode placed in the atrium, and sensing a cardiac signal via a defibrillation electrode. The method further comprises the step of transferring the sensed signals from the heart to a sensing circuit.